In performing radiation therapy of cancer, in order to confirm energy and shape of a radiation beam, such as an X ray, an electron beam, a particle beam, etc., before beams are irradiated onto a patient, it is necessary to measure the dose distribution in a water phantom simulating human body. In order to adjust a radiation irradiation system such as an accelerator, and confirm the beam energy distribution and a shape which is different between each patient, routinely, it is necessary to measure the dose distribution as quality control of radiation beam.
In conventional absorbed dose distribution measurement as disclosed in Patent Document 1, by using a water tank simulating human body and one ionization chamber at which a driving system is provided so as to be able to change a position in water, by scanning the ionization chamber, the dose distribution in water which is generated by irradiation of radiation is measured. As a result, even for measuring the dose distribution once, tremendous time and labor is required. Further, as every time a beam condition is changed, confirmation by the dose distribution measurement is required, there is a limit of number of patients to be able to treat by one irradiation device, that is, there is a limit to improve operating ratio of therapy system
In order to solve the above-mentioned problems, as a device which can measure the dose distribution in a short time, radiation detectors or dose distribution measuring devices having various forms have been proposed. For example, in Patent Document 2, in order to measure the beam profile, a configuration of plate-shaped electrode-intermediate layer (with air hole of ionization chamber)-split electrode was proposed. In addition to the lateral direction distribution, in order to artificially measure the depth direction distribution, an energy compensation plate having an energy attenuation unit is provided at an upstream side.
Further, in Patent Document 3, it is disclosed a configuration to measure the three-dimensional distribution of radiation wherein a plurality of ion chambers at which split electrodes are arranged in a cross shape are laminated, radiation stopping power in a thickness direction of each ion chamber is made to be same as that of radiation stopping power of water having same thickness and a water phantom is not used.
In ionization chambers comprising split electrodes which are disclosed in Patent Document 2 or Patent Document 3, in a case where high speed and high resolution is intended to concurrently achieve, volume per one cell of electrode is necessarily small, consequently, sufficient signal output cannot be obtained. That is, the signal-to-noise ratio cannot be increased, and it is difficult to secure precision. As a way to counter the above-mentioned problem, in Patent Document 4, a configuration to increase the signal-to-noise ratio, wherein each dummy signal line is arranged in the vicinity of each signal line which is connected to a split electrode, by obtaining difference between an output signal of a signal line and an output signal of a dummy signal line, is disclosed.